Richard Trevithick's high pressure rotative steam engine built specifically to thresh grain represents the first time that steam power was applied to agricultural uses. Sir Christopher Hawkins, landowner in Cornwall, was the first to purchase one, for his estate Trewithen. The original, one of many to come, has survived and is preserved in the Science Museum in London.

The single-acting engine was built at Harvey & Co. in Hayle. It has a deep piston and an open-topped cylinder, 230mm in diameter, with the steam distributed by a three-way stopcock. It had a power output of 3kW and worked at 30 strokes per minute. The boiler converted 41 litres of water into steam per hour and it was capable of six hours’ continuous operation before the water needed replenishing.

Trevithick (1771-1833) coupled his engine to what he called a ‘thrashing machine’, a rotating drum 910mm in diameter that separated the grains from the stalks of crops. The first one was bought for £90 by Hawkins (1758-1829) and used on his estate at Trewithen near Probus. It replaced a cattle-driven mill used previously at the estate’s farm.

Hawkins was a Cornish MP, whose family owned Trewithen House (SW912475) from 1715 onwards. The house was remodelled by architects James Gibbs (1682-1754) and Thomas Edwards (d.1775) in the 1730s, and by Sir Robert Taylor (1714-88) in the 1760s. It is built of granite and Pentewan stone with hipped slate roofs, and is Grade I listed. Trewithen means "house in the spinney" in Cornish.

The threshing machine was installed at Trewithen by early February 1812 and at a trial on 20th February, three independent witnesses stated that — "a fire was lighted under the boiler of the engine five minutes after eight o'clock, and at twenty-five minutes after nine the thrashing mill began to work, in which time 1 bushel of coal was consumed”. In this case a bushel was about 38kg (84lb), though it varied from place to place. Once working, the mill threshed 1,500 sheaves of barley in 4 hours 45 minutes without using all the water in the boiler.

The fact that the boiler was heated by a fire underneath it, rather than by an internal fire tube, indicates that the original boiler was a ‘pot boiler’. The boiler now attached to the machine in the Science Museum is a cylindrical boiler with internal heating, of the type Trevithick invented about this time — which came to be known as the ‘Cornish boiler’.

By 10th March Trevithick was improving his first design and was “building a portable steam-whim on the same plan, to go by itself from shaft to shaft”. It had a Cornish boiler and weighed some 1.5 tonnes, with a power “equal to twenty-six horses in twenty-four hours” (19.4kW). According to his son and biographer Francis Trevithick (1812-77), writing in 1872, he advertised this engine for sale at £105, with a smaller version weighing 760kg for £63.

On 26th April, Trevithick wrote to Sir John Sinclair (1754-1835) of the Board of Agriculture that, “It is my opinion that every part of agriculture might be performed by steam, carrying manure for land, ploughing, harrowing, sowing, reaping, thrashing, grinding, and all by the same machine, however large the estate”.

The thrashing machine was so successful that Francis Basset, Lord de Dunstanville (1757-1835), ordered a similar engine for his estate at Tehidy House (SW654434) in December 1812. The stationary device was installed in a barn. The boiler is the only part of it to survive and is also preserved at the Science Museum — it too is a pot boiler, made from wrought iron with cylindrical sides and a spherical base topped by a flat cast iron lid.

Further orders for agricultural engines followed, including one from Padstow and one from Bridgnorth in Shropshire. The Trewithen engine was in continuous use from February 1812 until 1879, when it was moved to Kilburn. The boiler was replaced some time between these dates, possibly in 1854.